Measurement or imaging of minute luminescence or fluorescence emitted from the body has become increasingly active in the field of medicine or biotechnology in recent years.
In the medical or security field, a technique of converting a small quantity of X-rays transmitted through the body into visible-level photons through a scintillator and detecting them to perform a transmission imaging has been industrialized. In addition, in the medical or security field, a technique (for example, a SPECT or a PET) of converting γ-rays generated from a small quantity of radiation material injected into the human body into photons through a scintillator has been industrialized.
In imaging in such a field, a photon counter is used for a very small amount of light.
Typically, a photon counter is a single device using an avalanche diode or a photomultiplier tube.
This photon counter generates a voltage pulse at the output by converting photons incident on the light receiving surface into photoelectrons, accelerating the photoelectrons with a high voltage, and multiplying them by generation of secondary electrons by collision.
The number of pulses is measured by a counter device connected to the device all the time.
While the photon counter has high measurement accuracy allowing detection in units of one photon, the system is expensive and the dynamic range for measurement is also narrow.
Usually, the number of photons which can be measured by one photon counter is about 1 million to 10 million for 1 second.
On the other hand, for imaging in a range of a relatively large amount of light to be measured, a photodiode and an analog-to-digital (AD) converter are used.
The photodiode accumulates electrode charges photoelectrically converted and outputs an analog signal. This analog signal is converted into a digital signal by the AD converter.
Problems in such imaging are noise caused by transmission of an analog signal and the conversion rate of the AD converter.
In order to detect a small amount of light, it is necessary to suppress noise and also to increase the number of bits in AD conversion for fine chopping. However, in order to do so, a very high-speed AD converter is necessary. In addition, if this is made to have a large number of pixels in order to improve the resolution in imaging, the system size for AD conversion is significantly increased.